1. Field of the Invention
The present invention relates to a semiconductor light-emitting device.
2. Related Background Art
Recent years have seen various proposals of semiconductor light-emitting devices made from InGaAlP-based materials, which emit light in a visible region and are widely used in displays and the like. Conventional semiconductor light-emitting devices are provided with, for example, an InGaAlP-based double heterojunction structure obtained by causing an n-type clad layer, an active layer and a p-type clad layer to grow epitaxially on an n-type GaAs substrate subsequently, an n-side electrode is formed on the bottom surface of the n-type GaAs substrate and a p-side electrode is provided in a contact layer on the p-type clad layer.
By selecting the band gaps and lattice constants of the active layer and the n-type/p-type clad layers which form this double heterojunction structure optimally according to design values, it is possible to confine carriers and to cause light to be emitted with a desired wavelength in a visible-ray region in increased efficiency.
For example, when the composition of the active layer which undergoes epitaxial growth is expressed by In0.5 (Ga(1-x)Alx)0.5 P and the composition of the n-type/p-type clad layer is expressed by In0.5(Ga(1-y) Aly)0.5 P, light emission from the red color band to the green color band can be obtained by appropriately selecting the amount of X or Y.
A GaAs substrate, which is the commonest in terms of the ease with which substrate procurement, lattice matching, etc. are performed, is used in InGaAlP-based double heterojunction semiconductor light-emitting devices. However, because the band gap wavelength of GaAs is 0.87 μm, the absorption coefficient of what is called visible light of not more than 0.87 μm becomes large and hence for a visible light semiconductor light-emitting device, about half the emitted light is absorbed in a GaAs substrate and luminance decreases.
In order to prevent the absorption of emitted visible light by a GaAs substrate, it is necessary only that a material transparent to visible light be used in the substrate. GaP is available as a general transparent semiconductor material. However, because a GaP substrate cannot ensure lattice matching with an InGaAlP-based material, it is difficult to cause good epitaxial crystals to grow. For this reason, there has been proposed a semiconductor light-emitting device which is fabricated by bonding together an InGaAlP-based epitaxial layer which has grown on a GaAs substrate and the GaP substrate directly in the form of wafers and removing the GaAs substrate thereafter.
With this semiconductor light-emitting device, the use of the bonded transparent GaP substrate enables the absorption of the emitted visible light to be suppressed and, therefore, a decrease in luminance can be prevented compared to a case where a GaAs substrate is used. Furthermore, there have been proposed semiconductor light-emitting devices having a substantially spherical shape as a general which are constituted by a substantially semispherical transparent p-type GaP bonded substrate, a substantially semispherical n-type GaP bonded substrate, and a light-emitting diode layer (corresponding to a light-emitting layer forming portion, which will be described later) formed from an epitaxial layer sandwiched between the two in order to increase luminance, i.e., the light extraction efficiency (refer to, for example, the Japanese Patent Laid-Open No. 2002-190619 (page 4, FIG. 1)).
In this disclosed semiconductor light-emitting device, because the ratio at which the emitted visible light is absorbed by a substrate is very low and because of the spherical outer shape, it is expected that the ratio at which the visible light which is emitted in the middle part near the center of the sphere can be taken out to the outside of the semiconductor light-emitting device also increases. However, because in the case of the light which is emitted in the light-emitting layer present near the spherical surface off the center, the ratio at which an angle at which the light becomes incident on the spherical surface deviates greatly from 90 degrees increases, the light which can be taken out to the outside of the semiconductor light-emitting device decreases. Specifically, although the ratio at which an inputted current is used for light emission in the middle part of the sphere is high from where the visible light can be taken out to the outside with good efficiency due to the spherical outer shape, the current also flows into the light-emitting layer present near the spherical surface from which the light cannot be taken out to the outside with good efficiency even with the spherical outer shape, thereby posing the problem that luminance is not insufficient for the inputted current. In other words, the problem was that the quantity of light which is capable of being taken out is small for an inputted current.